There are circuits or devices using integrated circuits which require circuit parameters to vary in a controlled manner to compensate for variations in the signal input. For example, repeaters in telephone systems must deal with variable line lengths, i.e., variable distances between repeaters. The amplitude of the regenerated signal depends upon the distance between repeaters, and the circuits which regenerate the signal must compensate for variable line lengths. As another example, the signal received directly from a magnetic disk as the disk is read typically has both pulse shape and amplitude variations. The circuit which processes this signal must be able to compensate and correct these variations. The parameter that is frequently varied, in the examples described as well as in other situations, to compensate for such variations is a resistance.
A common choice of a variable resistance is the source/drain resistance of an MOS field effect transistor operating in the triode region. In this region, the FET transistor behaves approximately like a linear resistance whose value is a function of gate voltage. The range of the gate voltage is bounded on the upper end by the supply voltage and on the lower end by the minimum overdrive (above the threshold voltage) required to keep the device in the triode region. This limits the range of resistances obtainable from the transistor. In fact, a typical range is only a factor of 3. However, for many applications the resistance must be varied over a substantial range; for example, an order of magnitude or more, that is significantly larger than is the linear region of a single field effect transistor. This wide requirement arises from the fact that the resistance has to compensate not only for the input variations, but also for its own variability due to processing and ambient temperature variations.
One way to increase the range of available resistance is to connect a plurality of transistors in parallel. The transistors have different sizes and different control voltages; that is, they turn ON at different voltages. The individual control voltages are derived from a single voltage. As this voltage increases, the first transistor turns ON and the equivalent resistance of that transistor decreases until it reaches the limit determined by its maximum gate drive. At this point, the second transistor turns ON and the described process repeats.
However, the circuit with parallel connected transistors just described will work properly only if there is a smooth transition from one transistor to the next transistor. That is, a transistor should turn ON at the point where the resistance of the previous transistor levels off. This is possible if the transitions are substantially independent of device processing and operating temperature. This, in turn, requires that the individual control voltages for each transistor be generated properly.